Softness is a desired property in tissues. Perceived softness correlates with properties of weak strength, enhanced bulk, and surface smoothness or texture. Methods of making soft tissue and towel are known and include, for example, Yankee creping, throughdrying, fabric creping, shoe pressing, and others. Some effects of such processes are to inhibit the formation of inter-fiber bonds, such as hydrogen bonds, as the sheet is dewatered, as well as to break up the bonds that have formed in the sheet as a result of the machine design.
Although the resulting tensile strength of a paper sheet after formation may not be fully understood, a number of theories provide reasonable models. According to one model (described in Page, D. H., “A Theory for the Tensile Strength of Paper,” PAPRICAN, PPR-7, July 1968), the tensile strength of a given population of fibers and a given paper machine design can be explained by the relative bonded area (RBA) of the fibers in the sheet. The RBA is a function of the number of inter-fiber bonds that form during the formation, handling, pressing and drying of the paper sheet. The strength of a wet web of cellulose fibers is initially low. As water is removed from the web, water molecules can form bridges between hydroxyl groups in adjacent fibers. As more water is removed, capillary forces (“Campbell Effect” forces) can draw the fibers close enough so that a hydrogen bond can form between fibers, giving the web dry strength. In another model (described in Tejado, A. and van de Ven, T. G. M., “Why Does Paper Get Stronger as it Dries?” Materials Today, September 2010, Volume 13 Number 9), surface tension and wetting forces also contribute to wet strength and formation of hydrogen bonds as the paper web dries. Similar forces occur in the hollow lumen of cellulosic fibers, which can cause them to collapse as water is removed and become flat and ribbon-like.
Other aspects of paper manufacturing can affect tensile. For example, pressing increases the tensile strength of a wet paper web by both removing water from the matrix and by bringing the fibers closer together so that fiber to fiber bonding is promoted. A papermaking process is described in U.S. Pat. No. 3,301,746 to Sanford et al., which eliminates wet pressing and thus aims to avoid fiber-to-fiber bonding and increase the softness, bulk and absorbency of a tissue sheet.
Another method used to increase softness is addition of chemical debonders to the cellulosic fibers during production. Chemical debonders inhibit the ability of fibers to form hydrogen bonds and therefore results in a reduced tensile strength.
Conventional wet pressed machines (CWP) utilize a pressing step to increase the solids content of the sheet as it is transferred to the Yankee drying cylinder. The bonds generated in the sheet by the pressing step are then disrupted by a combination of chemical debonder addition and creping the sheet off the Yankee dryer.
In addition, many modern sheet machines use “through air drying” (TAD) to reduce strength and increase bulk. TAD minimizes hydrogen bond formation in the sheet by removing water from an un-pressed wet web utilizing combinations of vacuum, steam and hot air and provides a reduced basis weight at a given bulk level. TAD provides a fiber cost savings over a CWP machine but requires a higher energy cost to thermally remove the high levels of water in the unpressed sheet.
Fabric creping (FC) processes increase the bulk and softness compared to CWP and provides lower energy costs than TAD. Chemical debonders may be used to increase the softness of tissues made by CWP and FC methods. However, chemical debonders may not be able to overcome the advantage of higher bulk at a given basis weight of TAD.
Although chemical debonders and TAD technology provide desirable tissue papers, these processes are expensive. Further, tissue paper production with TAD technology has an inherently high operating cost because of high energy input requirements.
The potentially detrimental impacts of air in the wet zones of a papermaking process are known. For example, as described in Turnbull, R. B., Jr., “Deaerator Design for Paper Machines,” Pulp and Paper Manufacture, Volume 6, Stock Preparation, TAPPI 1992, air in the formation zone and wet areas of a papermachine can result in poor formation, poor drainage, and runnability issues. Therefore, various approaches have been developed to mitigate air in the wet zones of the papermaking processes. One such approach, described in U.S. Pat. No. 5,308,384 to Kapanen et al., attempts to improve papermaking stock quality by initially de-aerating the stock.
Based on the foregoing, there still exists a need for a method for making a bulky, low strength tissue paper at reduced operating costs, compared to conventional methods, with low levels of chemical debonders or without chemical debonders. Accordingly, it is to solving this and other needs the present invention is directed.